In conventional data delivery systems, a delivery apparatus divides the contents data to be delivered into blocks, or unit data, of a predetermined size and sequentially transmits the contents data to a communication terminal on a unit data by unit data basis in a communication enabled state. Then, the communication terminal sequentially receives the unit data transmitted from the delivery apparatus and stores the block IDs fitted to the received respective unit data.
If the delivery is suspended because of a communication disabled state that arises before all the contents data are transmitted from the delivery apparatus, the communication terminal notifies the delivery apparatus of the block IDs of the unit data it has already received when the communication enabled state is restored. Then, the delivery apparatus identifies the blocks that have already been delivered and the blocks that have not been delivered yet out of the contents data according to the block IDs and transmits the unit data of the delivered blocks that have not been delivered yet to the communication terminal. With this arrangement, the delivery apparatus can practically avoid delivery of same blocks in duplicate and reliably transmit the blocks that have not been delivered because of the suspension of delivery to the communication terminal.
When the delivery apparatus delivers the contents data to the communication terminal for a charge, it executes an accounting process after the completion of the transmission of all the contents data to the communication terminal. Then, the delivery apparatus transmits a message to the communication terminal, notifying that the delivery of the contents has been completed and the accounting process has been properly executed (see, inter alia, Patent Document 1: Jpn. Pat. Appln. Laid-Open Publication No. 2002-135350 (pages 1, 6 and 11, FIGS. 1 and 17).
When the delivery apparatus of the data delivery system having the above described configuration delivers contents data for a charge, the delivery apparatus may transmit contents attribute information to the communication terminal after the transmission of the contents data for the purpose of altering the attributes relative to reproduction of the contents data in order to protect the copyright of the contents data. In other words, the communication terminal can reproduce the contents data according to what the contents attribute information tells only when it receives the contents attribute information along with the contents data. As for the delivery apparatus, when it transmits contents data to the communication terminal as charged service, it executes the accounting process while it is transmitting the contents attribute information to the communication terminal.
However, if the communication line between the delivery apparatus and the communication terminal is disconnected probably because of stoppage of power supply while the communication terminal is receiving the contents attribute information from the delivery apparatus of such a data delivery system, the copyright owner of the contents data can suffer economic losses when the transmission of the contents attribute information is completed but the accounting process is interrupted. Additionally, there can arise a situation where the user suffer economic losses because the accounting process is completed although the reception of the contents attribute information is not completed yet because of the disconnection of the communication line in such a data transmission system.
To date, when the communication line is disconnected while the communication terminal is receiving contents in such a data delivery system, it is not possible for the communication terminal to know if the accounting process is interrupted because of the disconnection of the communication line or if the accounting process is completed although the communication line is disconnected. Thus, there arises a problem that the communication terminal cannot properly handle such a situation when the communication enabled state is restored from the disconnection of the communication line relative to the delivery apparatus.